[Sorcery] Bridging spell

[Anders]

Need help with another Sorcery spell. I plan on making these available as PDFs in the GURPS Resources page one day (minus any spells I've swiped from published sources).

This spell creates a bridge made from shimmering blue magical energy emanating from the caster's feet. The bridge can be up to 3 yards wide and 50 yards long and supports up to 300 lbs. (more for extra levels?).

Create? Telekinesis? Walk on Air?

[Humabout]

I'd think Create Energy would cover this.

[Anders]

And such created energy could support 10 x levels squared lbs.? Sounds interesting.

[Varyon]

Only being able to make a magical bridge sounds about right for the "specific item" category, putting this at [5] per level. The issue we'll run into is how to represent the bridge's size and carrying capacity. Powers has Create Energy be based on kJ. If our bridge were straight up (making it more like a ladder), it would be able to produce potential energy (in kJ) equal to its carrying capacity (in kg) multiplied by the force of gravity (in N/kg) and its total height (in m). For GURPS, this works out to 20*m*h, where m is effective (once you factor in gravity) weight in lb and h is the height of the ladder. For a horizontal bridge, use 10*m*l instead (based roughly on projectiles using twice distance when fired vertically up), where l is length. This would be appropriate for a bridge that is 1 yard wide - wider bridges multiply effective height by width in yards. Powers has Create Energy produce up to 1000*(level squared) kJ. Combined with the previous, that means 100*(level squared) = m*l*w, where you'd fill in what you need and solve for the rest. If you need a 1-yard wide bridge that is 5 yards long, you'd solve for m - m=20*(level squared). A level 1 bridge could thus handle 20 lb, a level 2 could handle 40, and so forth.

It's probably fine for the bridge to arch - assume the bridge can be up to half as tall as it is long. If you need a steeper bridge, that should be possible, except you use twice its vertical height rather than its length to determine how much it can carry.

For your example of a 3x50 yard bridge that can support 300 lb, that's (level squared) = 300*3*50/100 = 450, which requires 22 (!) levels of Create Bridge to pull off. As a Sorcery spell, that's a base cost of [94], requiring Sorcery 9 to buy as a Known Spell, where it will cost [19]. Technically your bridge should vanish in 10 seconds unless stabilized, but I'd just say you can only create one bridge at a time and must stay within, say, 10 yards of it in order for it to not disappear. This same amount of Create would build a bridge of more mundane construction that weighed 4,840 lb, or around 2.5 tons. I've no clue how that would stack up, so instead let's use an existing bridge and see what it would cost to Create it, and then see how it stacks up to my suggestion. The Golden Gate Bridge weighs 1,774,000,000 lb, is 30 yards wide, and is right around 3,000 yards long. It can apparently hold up to 35,924,000 lb (4,000 lb per foot). Creating it would require 13,320 (actually, something more like 13,319.15) levels of Create Suspension Bridge. That many levels of Create Bridge, above, and with those dimensions would yield a bridge that could hold up to 197,111 lb. So, in a TL7 or TL8 setting, it might be appropriate to use something like 15000*(level squared) = m*l*w instead (which would yield a maximum load of just under 30,000,000 lb), in which case your 3x50x300 bridge would only require 2 levels of Create Bridge, making it fall into the "Cantrip" category. If you want some TL scaling, it might be appropriate to have the multiplier be TL dependent - if we use 15000 for TL 7, that implies 1000 for TL 0, 1500 for TL 1, 2000 for TL 2, 3000 for TL 3, 5000 for TL 4, 7000 for TL 5, 10000 for TL 6, and so forth. That makes the cost of the spell TL-dependent. If we really wanted to, we could instead assume my 100 multiplier is appropriate for TL 1 and 10000 is appropriate for TL 7 and scale from there instead - +12 SSR in 6 steps implies +2 SSR per step, so that implies 50 for TL 0, 100 for TL 1, 200 for TL 2, 500 for TL 3, 1000 for TL 4, 2000 for TL 5, 5000 for TL 6, 10000 for TL 7, 20000 for TL 8, and so forth. That puts the Golden Gate Bridge as right between TL 7 and TL 8, which kind of makes sense - it was built in TL 7 and renovated in TL 8.

[Anders]

This is for TL4. If that helps.

Also, if it's (level squared), shouldn't a level 2 energy bridge be able to hold 80 lbs.?

[Varyon]

A simplified version of what I said is this - "mass" of the bridge is equal to 100 * (level squared). Divide this by length and width to determine the weight it can handle. Optionally, this weight is per yard of width - a bridge that can handle 300 lb and is 3 yards wide can actually handle up to 900 lb at a time, provided it's spread out over those 3 yards (say, 3 rather encumbered people walking side by side). You may replace the 100 multiplier by a TL based factor, as noted in the above post.

Originally Posted by Anders

This is for TL4. If that helps.

The multiplier is up to you. Does it make more sense to you as a [94] (x100), [30] (x1000), or [13] (x5000) spell (considering these require Sorcery 9, 2, and 1, and end up costing [19], [6], and [3] as Known Spells, respectively).

Originally Posted by Anders

Also, if it's (level squared), shouldn't a level 2 energy bridge be able to hold 80 lbs.?

D'oh! For some reason I went from a multiplier of 20 to one of 10 between those two examples.

[Lia Valenth]

Originally Posted by Varyon

...
For your example of a 3x50 yard bridge that can support 300 lb, that's (level squared) = 300*3*50/100 = 450, which requires 22 (!) levels of Create Bridge to pull off. As a Sorcery spell, that's a base cost of [94], requiring Sorcery 9 to buy as a Known Spell, where it will cost [19]...

This is a well done idea, although it does show a problem I have with Create. So, to make sure I understand, you used Create (Energy Support) 22 [110] and added Sorcery, -15% [96]. This makes sense, but would definitely need more limitations to be reasonable.

I created a similar ability using
Innate Attack 1d Cr (AoE X, +(x*50)% ; Persistent, +40% ; Wall {rigid}, +60% ; Extended Duration x10 {100 seconds}, +40%) [14.5] +2.5pts/level of AoE.
This allows the user to create a wall of any shape 3 yards long by 1 yard with wide for each yard of radius with 3 DR and 1 HP per yard. Unless I am mistaken, For the 3yd x 50yd example this would need a 50-yard radius, 6 levels of AoE, for a total of [27]. I am not sure how much this would support though (probably not much).

I used Innate Attack with Wall because it is much, much cheaper than Create for this type of effect.

[A Ladder]

Gurps’ modus operandi for creating powers is more of “create the effect, any visual representation is then fluff”. I would go with Walk on Air and then just have the description be “Shiny blue bridge of energy” (with the Visible limitation), because the spell is really just allowing people to Walk on Air above a certain area.

How I would build it:
Create Bridge
This spell creates a bridge made from shimmering blue magical energy emanating from the caster's feet. The bridge can be up to 5 yards wide and 64 yards long and supports up to 500 lbs. This spell can only be cast on fixed points on the ground and lasts one minute.

Affliction (Bridge +300%; Area of Effect: 32 yds, +250%; Variable, area +5%; No Signature +20%; Increase ½ D x10 +15%; Sorcery -15%; Fixed Duration +0%; Visible -10%; Requires "Ground" -20%) [65]

Bridge: Affliction (Walk on Air +200%; Trigger: When people enter the Bridge Area +50%; Fixed Duration +0%; Accessibility: Only while in the Bridge Area -20%; Only 500 lbs of weight total in the Bridge Area -20%; Magical -10%) [30]

The thing I don't know how to price would be the the weight accessibility. But I figure that 300 lbs should cover about 50% of the circumstances that people would want to cross a magical bridge. If someone else knows better, please adjust.

EDIT: On second thought, the 300 lb weight limit is crippling. You can only have two peasants cross the bridge at a time. A Knight in full plate (or even mail armor) couldn't hope to cross the bridge. Forget about bringing your big strong Barbarian friend. I'm adjusting to 500 lbs.

[ericthered]

"Only while on bridge"can be rephrased as "reduced duration 1/100" for -40%

[PK]

One easily overlooked way to create force walls in GURPS is to use the Wall enhancement on Innate Attack. Specifically, you could do this as Crushing Attack (Area Effect; Wall) to create an actual, physical bridge with HP and DR.

The only downside is that there's no specific correlation between "bridge HP/DR" and "bridge weight limit." However, you could approximate this by assuming that even the bumpiest ride over a bridge will cause no more of a downward impact than a 1-yard fall would (in other words, a truck driving over a bridge is certainly going to stress it less than if you dropped that truck onto the bridge from a 1-yard height!) and making sure the DR can absorb this. It's intentional overkill, so the GM can feel confident in never needing to worry about it.

Since a 1-yard fall is Move 5, assume that vehicles "inflict" (HPx5)/100 dice, aka HP/20 dice, which multiplied by 3.5 is about HP/5.7 points of damage. Since each level of Crushing Attack gives DR 3 (and 5.7x3=17) that means the Crushing Attack level (CAL) must equal HP/17, or in plain English, one level of Crushing Attack per 17 HP of vehicle weight. Because the stress is going to almost always come from all vehicles traveling carefully over the bridge, it's fair to lump their weight together.

Technically the travelers may be some combination of living, unliving, and homogenous, but for simplicity just use the "Unliving/Machine" column on p. 558, and you get this:

CAL -- Total Weight Supported
1 -- 76 lbs.
2 -- 614 lbs.
3 -- 2,072 lbs.
4 -- 4,913 lbs.
5 -- 9,595 lbs.
6 -- 16,581 lbs.
7 -- 26,330 lbs.
8 -- 39,304 lbs.
9 -- 55,962 lbs.
10 -- 76,765 lbs.

Note that the formula for deriving weight is Weight in lbs. = (HP/4)^3 = (CAL*4.25)^3 (round down, or just round to 2 or 3 significant digits).